How to scale analog inputs and outputs – PLC scaling examples
Analog I/O whether it is 4-20mA or 0-10 V are connected to pressure transducers, level probes and other analog devices. As PLC programmer or control engineer you will need to get some knowledge about scaling analog signals from plc I/O modules. Scaling helps to convert analog value to measure units ( pressure, level etc.).
Most industrial applications usually require measurements in engineering units, which provide more meaningful data. We can achive this by using the conversion formula shown: Example by AutomationEngineering.
Time for example
If we want to measure level in tank from 0 to 65 meters then our L = 0, H = 65 max value from our analog card 32000 ( in engineering units ). Current value A from analog input is 13454. Now if we calculate it using our equation we will get current level of 23.7 meters.
Now situation looks slightly different with Analog Output.
Your PLC program has to calculate the digital value to send to the analog I/O module. There are many ways to do this, but most industrial applications are understood more easily if you use measurements in engineering units. We can do this by using the conversion formula shown.
Consider the following example with level measurement
We know there is 38 meters of liquid in tank. Our H = 65m L=0m. Max analog output value is 32000. If we use our calculation we will get 18707 which we are going to send to our Analog Output.Many Programmable Logic Controllers have ready to use instruction for scaling.
S7-200 Siemens PLC scaling the analog input in the output data format REAL (S_ITR)
The S_ITR function permits you to convert the analog input signal into a normalized value between 0.0 and 1.0 (type REAL)
The following table explains the abbreviations used:
|Ov||Scaled output value (Output value)|
|Iv||Analog input value (Input value)|
|Osh||Upper scale limit for the scaled output value (Output scale high)|
|Osl||Lower scale limit for the scaled output value (Output scale low)|
|Ish||Upper scale limit for the scaled input value (Input scale high)|
|Isl||Lower scale limit for the scaled input value (Input scale low)|